48. Surface tension

In the world of everyday life that surrounds us, along with gravity, elasticity and friction, there is another force that we usually ignore. This force is relatively small, its actions never produce eye-catching effects. Nevertheless, we cannot pour water into the glass, nothing can be done with any liquid without triggering the forces we are now talking about. These are the forces of surface tension.

We are so used to the effects caused by surface tension that we do not notice them if we do not have fun launching soap bubbles. However, these effects play a significant role in nature and our lives. Without them, we couldn't write in ink. An ordinary brush would not scoop paint out of a can, but would immediately put a large stain. We couldn't wash our hands, foam wouldn't form. A slight rain would have soaked us through, and the rainbow would have been impossible to see in any weather. The water regime of the soil would be disturbed, which would be disastrous for the plants.

The easiest way to capture the nature of surface tension forces is to observe a drop in a poorly closed or faulty hydrant. Look closely at how the drip grows, the neck is narrowed, and the drip tears off. It does not take much fantasy to imagine that the water is as if enclosed in an elastic bag and this bag breaks when the weight of the water exceeds its strength. In fact, of course, nothing but water in the drop, but the surface layer of water itself behaves like a stretched elastic film.

Soap bubble film makes the same impression. It's like the thin, stretched rubber of a baby ball. Carefully place the needle on the water surface. The surface film will bend out and prevent the needle from drowning. For the same reason, lightweight water striders can glide quickly on the water surface like skaters on ice.

The deflection of the film does not allow water to pour out, which is gently poured into a sufficiently frequent sieve. The fabric is the same sieve formed by a weave of threads. Surface tension makes it very difficult for water to seep through, so it does not get wet instantly.

In its effort to shrink, the surface film would have given the liquid a spherical shape if there was no attraction to the Earth. The smaller the droplet, the greater the role of surface forces compared to volume forces (gravity). Small droplets of dew are therefore close in shape to the ball. A free fall results in a state of weightlessness (zero gravity) and therefore rain droplets are almost exactly spherical. Due to the refraction of the sun's rays, a rainbow arises in these droplets. If the droplets were not spherical, there would be no rainbow.
In a spaceship in a state of zero gravity, large masses of liquids take the spherical form.

The appearance of surface tension forces can be explained as follows. If a large group of individuals has the property of attracting each other or if individuals rush to each other at will, the result will be the same: they will gather in a lump like a bee swarm. Each individual strives inside this lump, and the surface of the lump shrinks, approaching a sphere.

The molecules of water or other liquids that are attracted to each other tend to get closer. Each molecule on the surface is attracted by the other molecules inside the liquid and therefore tends to sink deeper. Since the liquid flows because of the jump of molecules from one sedentary position to another, it takes the form in which the number of molecules on the surface is minimal. And the minimum surface at the given volume has a ball. The surface of the liquid is reduced, and it is perceived as surface tension.

Here we find that the origin of surface forces is quite different from the elastic forces of a stretched rubber film. And it's true. As rubber shrinks, the elastic force weakens, but the surface tension forces do not change as the surface of the film shrinks, because the density of the liquid, and therefore the average distance between the molecules on the surface, does not change.

Therefore, the appearance of surface forces cannot be explained as easily as the elastic forces, where everything is connected with the change of distance between the molecules. Here, it is more and more complicated, because the surface tension forces are revealed by complex rearrangement of the shape of the whole liquid without changing its volume.